A voltage-controlled oscillator (VCO) device consists of two components: an HBT and a varactor. A varactor is an electronic device whose capacitance can be varied by changing the applied voltage. They are applied mainly in various tuning circuits. In conventional compound semiconductor manufacturing processes, the base-collector junction of an HBT is usually utilized as a varactor. FIG. 1A is a schematic showing the cross-sectional view of a conventional varactor, consisting of a p-type doped layer 101, a first n-type doped layer 102, and a second n-type doped layer 103. Such a varactor structure can be formed by an HBT, in which the base layer (p-type), the collector layer (n-type), and the sub-collector layer (n-type) form respectively the p-type doped layer 101, the first n-type doped layer 102, and the second n-type doped layer 103. By this means, the fabrication processes for varactors are compatible with that for HBTs, which considerably simplifies the whole manufacturing process. In typical HBT devices, the n-type doping concentration in the collector layer is designed to have an increasing concentration from the base layer to the sub-collector layer. That is, the n-type doping concentration in the collector layer is lower near the base layer, as shown in FIG. 1A, such that the base-collector junction in an HBT can have a higher breakdown voltage and a lower leakage current. On the other hand, to produce a high capacitance ratio (C ratio) in a varactor, a large gradient in the doping concentration is essential. FIG. 1B shows a better design for a varactor, where a concentration gradient in the first n-type doped layer 102 with the highest concentration adjacent to the p-type doped layer 101 is introduced, by which a higher C ratio can be achieved. However, such a concentration gradient in the collector layer is detrimental for an HBT, since it will reduce the breakdown voltage and increase the leakage current. Therefore, a new epitaxial structure and fabrication process for monolithically integrated VCO devices are demanded.
In view of these facts and for overcoming the drawback stated above, the present invention provides an integrated structure of compound semiconductor devices, which includes optimized HBT and varactor.